This invention relates generally to condensate removal devices in gas piping systems, and more particularly to a modular steam trap for removal of water condensate from steam pipes.
Steam is an efficient and widely used heat transfer medium for transporting energy. An unavoidable by-product when using steam is liquid condensate (i.e., water) that forms when heat is transferred away from steam along pipes or at heat exchangers. When condensate collects inside pipes or other components, system efficiency is significantly degraded. Condensate also can cause a destructive water hammer, a shock wave that damages components and can cause serious injury to people nearby. Accordingly, condensate should be removed from steam systems as it forms.
A steam trap is a mechanical device used to drain condensate while retaining or "trapping" steam. Traps are typically positioned at natural low points in steam systems where condensate collects or ahead of control valves where condensate could impede proper valve operation. Most traps operate using the inherent difference in density between liquid and gas to separate the fluids. Ideally, each trap should be capable of draining a massflow, or load, of condensate that flows to its location in the steam system. Each trap should also be reliable in operation to avoid costly inefficiencies that arise when condensate collects or when live steam is released from a defective trap. Several types of steam traps are commonly available. Some are complex in design and subject to fail without frequent maintenance.
One type of trap that is economical and reliable is a fixed orifice trap. A relatively small hole or a tubular passageway in a trap permits condensate to drain through. These traps are comparatively inexpensive and there are no moving parts to corrode or fail. They are very effective in draining condensate while preventing release of live steam. The condensate flowing in a fixed orifice generally blocks entry of steam. However if steam does enter the orifice, it would be desirable to condense the steam into water to block entry of additional steam into the trap.
A drawback to fixed orifice traps is that they cannot accept large variation in condensate load. The diameter of the orifice is fixed, and therefore the capacity of the trap, which is proportional to area of the orifice and the flow velocity, is also substantially fixed. Orifices are sized to drain an expected load. The actual load, however, can increase by a factor of four or more if ambient temperature decreases, causing heat transfer rates from the steam to increase and causing formation of a larger quantity of condensate. In the past, this has been partially compensated for by over-sizing the orifice for the particular application. An over-sized orifice not only passes more load, but possesses a valuable secondary benefit of a greater ability to pass solid debris. Small deposits of corrosion or other particulate matter may become mixed within the flow of condensate and can clog the trap. There is less tendency for solid particles to lodge in an orifice or passageway that is relatively larger. However, a trap having an orifice that is larger than needed for ordinary loads tends to permit release of live steam and is inefficient.
A second type of trap is a thermodynamic or disk type trap. An obstruction comprising a flat disk is freely captured in the trap and is movable between a closed position in which the disk blocks flow of fluid through the trap, and an open position in which the disk permits flow of fluid. The disk may cycle between open and closed positions, and when in the open position the trap is capable of handling a greater quantity of condensate load than a fixed orifice trap. Condensate flow initially raises the disk open as it flows in. When steam arrives it changes the local pressure and lowers the disk, closing the trap, which stays closed as long as relatively higher pressure is maintained above the disk. At each cycle, there is an inherent time delay for closing the disk, as is common in thermodynamic traps, during which some live steam is released from the trap. So although the thermodynamic trap is beneficial in draining a large quantity of load, it has inherent inefficiency.
Typically it is not clear which type of trap is best suited for application at a location in a steam system. Fluid flow conditions, including pressure, temperature, condensate load, and amount of solid debris vary from one region of a system to another. Accordingly, different types of trapping modules may be more appropriate for placement in certain areas of the steam system. Unfortunately, knowledge of flow conditions is uncertain, and the conditions vary over time. In practice, many operators maintain a large and cumbersome inventory of several types of steam traps, and they choose one trap appropriate to estimated flow conditions. Operators may need to change steam traps because of altered or mistakenly estimated conditions. When an installed trap is removed and replaced, it often requires breaking a steam line, resulting in substantial downtime for the entire steam system.